1. Field of the Invention
This invention relates to a pattern forming method which is usable in the course of producing semiconductors such as ICs, producing liquid crystals or circuit boards such as thermal heads and lithographing other photofabrications, a resist composition for negative development to be used in the pattern forming method, a resist composition for multiple development to be used in the pattern forming method, a negative developing solution to be used in the pattern forming method and a rinsing solution for negative development to be used in the pattern forming method. In particular, it relates to a pattern forming method appropriately usable in exposure with the use of an ArF exposure device and an immersion type projection exposure device wherein far-ultraviolet light having a wavelength of 300 nm or less is employed as the light source, and a resist composition to be used in the pattern forming method, a negative developing solution to be used in the pattern forming method and a rinsing solution for negative development to be used in the pattern forming method.
2. Description of the Related Art
After the development of a resist for a KrF excimer laser beam (248 nm), use has been made of a so-called chemical amplification image forming method that is a resist image forming method for compensating for lowering in sensitivity caused by light absorption. In the positive type chemical amplification method for forming an image, for example, an acid-generating agent is decomposed upon exposure in an exposed part to form an acid. In baking after the exposure (PEB: post exposure bake), the acid thus generated is utilized as a reaction catalyst and thus an alkali-insoluble group is converted into an alkali-soluble group. Thus, the exposed part is removed by the alkali development to thereby form an image.
With the recent fine patterning in semiconductors, attempts have been made to shorten the wavelength of an exposure light source and elevate the numerical aperture (high NA) of a projector lens. At present, there has been developed an exposure device using an ArF excimer laser beam having a wavelength of 193 nm as a light source. It is widely known that such devices can be indicated by the following formulae.(Resolution)=k1·(λ/NA)(Focal depth)=±k2·λ/NA2 
In the above formulae, λ represents the wavelength of an exposure light source; NA represents the numerical aperture of a projector lens; and k1 and k2 represent coefficients relating to the process.
As a technique for elevating resolution, there has been known a so-called immersion method wherein the space between a projector lens and a sample is filled with a liquid having a high refractive index (hereinafter also called “immersion liquid”).
Concerning this “immersion effect”, the resolution and focal depth as described above can be indicated by the following formulae, wherein λ0 represents the wavelength of the exposure light in air, n represents the air refractive index of the immersion liquid, θ represents the convergence half angle of the light, and NA0 is referred to as sin θ.(Resolution)=k1·(λ0/n)NA0 (Focal depth)=±k2·(λ0/n)NA02 
Namely, the immersion effect is equivalent to using exposure light of the wavelength 1/n. In other words, the immersion makes it possible to elevate n-fold the focal depth in the case of using a projection optical system of the same NA. This is effective on any patterns and, moreover, can be combined with the super-resolution techniques under study, for example, the phase-shift method and the distortion illumination method.
To further elevate resolution, there have been proposed the double exposure technology and the double patterning technology which are regarded as techniques whereby resolution is elevated by lessening k1 in the above formulae concerning resolution.
To pattern an electronic device such as a semiconductor, it has been a practice to transfer a pattern of a mask or a reticle, in which the target pattern has been enlarged 4- to 5-fold, onto a substrate to be exposed such as a wafer with the use of a reducing projection exposure device.
With the recent fine patterning, however, there arises a problem in the existing exposure system that light interference occurs in adjacent patterns and thus optical contrast is reduced. To overcome this problem, attempts are made in these techniques to divide the mask design into patterns of a plurality of masks and separately expose the individual masks, thereby forming an image. In such a double exposure system, it is required to divide the mask design and combine these patterns again on a substrate to be exposed such as a wafer to thereby form an image. Therefore, the mask design should be divided in such a manner as ensuring the faithful reproduction of the pattern on a reticle.
JP-A-2006-156422 shows a case wherein the effect of this double exposure system on the transfer of a fine image pattern in a semiconductor.
Moreover, recent advances in the double exposure technology are reported in SPIE Proc 5754, 1508 (2005), SPIE Proc 5377, 1315 (2005), SPIE Proc 61531K-1 (2006) and so on.
In the case where a pattern is formed merely by applying an existing resist composition to an existing resist process, there arises a problem that neither a sufficient exposure margin nor a sufficient focal depth can be obtained since the pattern should be formed in the vicinity of the limit of resolution of the resist in the double exposure system.
Namely, no sufficient resolution performance can be established by applying a pattern forming process which includes coating a substrate with a resist composition containing a resin showing an increase in polarity upon exposure, exposing, and developing by dissolving an exposed part of the resist film with an alkaline developing solution as reported in JP-A-2001-109154, etc., or a pattern forming process which includes coating a substrate with a resist composition containing a resin showing an increase in molecular weight upon exposure, exposing, and developing by dissolving a non-exposed part of the resist film with an alkaline developing solution as reported in JP-A-2003-76019, etc. to the double exposure system.
As developing solutions for g-ray, i-ray, KrF, ArF, EB and EUV lithography, a 2.38% by mass aqueous alkaline developing solution of TMAH (tetramethylammonium hydroxide) is employed in these days.
As developing solutions other than the above-described one, for example, JP-A-2001-215731 discloses a developing solution for dissolving and developing an exposed part of a resist material containing a copolymer composed of an ethylene-based monomer and an acrylic monomer, which contains an aliphatic linear ether solvent or an aromatic ether solvent with a ketone (having 5 or more carbon atoms) solvent. JP-A-2006-227174 discloses a developing solution for dissolving and developing an exposed part of a resist material being degraded via cleavage in a polymer chain upon radiation, which is characterized by containing two or more of acetate, ketone, ether and phenyl groups and having a molecular weight of 150 or more. JP-A-6-194847 discloses a developing solution for developing an unexposed part of a resist material containing as the main component a photosensitive polyhydroxy ether resin obtained by reacting a polyhydroxy ether resin with glycidyl(meth)acrylate, which is characterized by using an aromatic compound having from 6 to 12 carbon atoms or a solvent mixture containing 50% by mass or more of an aromatic compound having form 6 to 12 carbon atoms as the developing solution.
However, these combinations of a resist composition with a developing solution merely provide a system wherein a specific resist composition is combined with a highly polar alkali developing solution or a developing solution containing an organic solvent with a low polarity to form a pattern.
That is, in a positive system (a combination of a resist composition with a positive developing solution), there is merely provided a material whereby a pattern is formed by selectively dissolving and removing an area with high photo irradiation intensity in an optical aerial image (photo intensity distribution), as shown in FIG. 1. In a negative system (a combination of a resist composition with a negative developing solution), on the other hand, there is merely provided a material whereby a pattern is formed by selectively dissolving and removing an area with low photo irradiation intensity.
The term “positive developing solution” as used herein means a developing solution by which an exposed part located at a definite threshold (shown by the solid line in FIG. 1) or above is selectively dissolved and removed. The term “negative developing solution” as used herein means a developing solution by which an exposed part located below the definite threshold is selectively dissolved and removed. A development step using a positive developing solution is called a positive development (also called a positive development step), while a development step using a negative developing solution is called a negative development (also called a negative development step).
JP-A-2000-199953 discloses a double development technique as a double patterning technique for improving resolution. In this case, a common image forming method by chemical amplification is employed. By utilizing the phenomenon that the polarity of a resin in a resist composition is elevated by photo exposure in an area with a high photo intensity and lowered in an area with a low photo intensity, positive development is conducted by dissolving a high exposure area of a specific resist film with a developing solution having a high polarity and negative development is conducted by dissolving a low exposure area thereof with a developing solution having a low polarity. More specifically speaking, an area wherein the exposure dose of irradiation light 1 is E2 or more is dissolved by using an aqueous alkali solution as a positive developing solution, while an area wherein the exposure dose is E1 or less is dissolved by using a specific organic solvent as a negative developing solution, as shown in FIG. 2. Thus, an area with medium exposure dose (E2 to E1) is left as a non-developed area and an L/S pattern 3 whose pitch is half that of the mask pattern for exposure 2 is formed on a wafer 4, as shown in FIG. 2.
In the above-described cases however, tert-butyl group is employed as an acid-decomposable group in the resin contained in the resist composition. Therefore, it is impossible to express a sufficient change in polarity for causing a difference in dissolution characteristics due to the chemical amplification reaction accompanying exposure.
Because of using a resin having a styrene skeleton as the resin in the resist composition, moreover, the low exposure area of the resist film has a high polarity. As a result, the development with the use of the negative developing solution proceeds at only a low development speed, which brings about a problem that the developing properties in using the negative developing solution are deteriorated.